1. Field of the Invention
This invention relates to automatic drive belt tensioning apparatus and, more particularly, to an apparatus of the type having a pivotable tensioner arm with an idler pulley thereon that is normally urged against an unsupported portion of a drive belt to create a predetermined tension thereon and absorb system vibrations imparted through the belt.
2. Background Art
Power transmission systems employing drive and driven pulleys, with an endless transmission belt extended therearound, are used in many different environments. In a simple system having a single drive pulley and a single driven pulley, it is common to employ an automatic belt tensioner having an idler pulley that is biased against an unsupported portion of the belt between the pulleys to maintain a constant tension on the belt. Conventional systems may use torsion springs or compression springs to impart the requisite force to the idler pulley.
The advent of the serpentine belt system in automobiles introduced new considerations for designers of belt tensioners. In such systems, an endless belt is trained in a circuitous path about a plurality of accessories, such as pumps, alternators, etc. The belt frictionally drives a plurality of pulleys associated with shafts on the accessories. For the accessories to operate consistently and effectively, it is necessary for the frictional drive force developed on the accessory pulleys to be relatively constant i.e. for the tension on the belt to remain constant.
The length of the drive belt in serpentine belt systems aggravates the problem of maintaining a constant tension. The tension on the belt is affected by all of the individual components and any irregularity in the operation of any one of the components may induce vibrations on the belt which, if not suppressed, might affect operation of the other accessories. Consequently, for a belt tensioner to be efficient in such serpentine systems, it is necessary that a tensioner be capable not only of maintaining a constant belt tension, but also of absorbing detrimental vibrations transmitted through the belt.
One system designed to accomplish this end is shown in U.S. application Ser. No. 421,259, assigned to Mitsuboshi Belting Ltd. This system employs a damping element which is seperately spring biased against a part of a pivotable tensioner arm carrying an idler pulley. While this system has proven effective in damping system vibrations and maintaining a constant belt tension, it has some drawbacks.
First, separate spring elements are used to bias the tensioner arm in a tensioning direction and the damping element against the tensioner arm. The tensioning spring is a torsion spring, which also tends to absorb some system vibration and avoids kickback of the tensioner arm when there is a slackened condition of the belt. A separate compression type spring is used to bias a frictional damping element against a part of the tensioner arm.
There has been an increasing trend in the automobile industry towards maximizing passenger compartment space and minimizing the compartment space required for the engine. With the premium on space in the engine compartment, it is therefore important that all engine components, to include the belt tensioner, be made as small as possible, without compromising the operation thereof. The inherent drawback with the above system is that certain configurations for the system may become quite large to accommodate the separate springs and other necessary elements.
Another problem with the above belt tensioner is that certain embodiments thereof may have the friction member keyed to a supporting shaft for the belt tensioner so that the friction member is slidable axially relative to the rotational axis for the tensioner arm, yet fixed against rotation about that axis. The need to place splines or other similar guide structure on the friction element, and that element against which it slides, complicates manufacture and may appreciably add to the cost of construction.
While the use of a deformable slide bearing between the tensioner arm and a support therefor to some extent damps undesirable vibrations, this slide bearing alone may not give sufficient stability to the mount of the tensioner arm which may result in an undesirable skewing of the tensioner arm relative to the support. The result is generally that the tensioner arm will not smoothly rotate on the support, which may affect operation of the system components.
Further, to limit binding between the tensioner arm and support a gap may be left between the bearing and one of the tensioner arm and support therefor. The gap also allows undesired canting of the tensioner arm relative to the support.